Adding a Target Line to Numbers

I would like to insert a target line into a chart I have created to track the number of steps I walk every day/month. By just inserting a line into the chart it doesn't replicate a target line because it doesn't move along with the scale changes. I haven't been able to find anywhere in Numbers where the insertion of a target line is referenced.

As an example if I set a target line at 10,000 steps and I walk 15,000 steps in a day, the scale readjusts however the line I inserted remains stationary. I assume Apple hasn't provided for this option. Can anyone help?